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LEUKEMIA

Pathology Assignment — Pharmacy Students

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1. Introduction

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2. Classification

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3. Pathogenesis

3.1 General Mechanisms

• Maturation arrest — cells are "frozen" at an immature stage and cannot differentiate
• Self-renewal — stem cell-like capacity for indefinite proliferation
• Resistance to apoptosis — escape from programmed cell death

1. Type I mutations — activate growth factor receptor signaling pathways (e.g., RAS, tyrosine kinases), promoting cell survival and proliferation
2. Type II mutations — disrupt transcription factors required for normal differentiation (e.g., PAX5, RUNX1, NOTCH1), causing maturation arrest

3.2 Specific Molecular Mechanisms by Type

Acute Lymphoblastic Leukemia (ALL)

• B-ALL (~85% of ALL): mutations in transcription factor genes — PAX5, TCF3, ETV6, RUNX1, BCR::ABL1, KMT2A, and PBX1 — required for B-cell differentiation
• T-ALL (~15%): mutations in NOTCH1 (50–70% of cases), a gene essential for T-cell development
• Chromosomal aberrations in ~90% of ALL: hyperploidy (>50 chromosomes) is common and confers better prognosis; hypoploidy carries worse prognosis
• The BCR::ABL fusion (Philadelphia chromosome) occurs in a subset of B-ALL ("Ph+ ALL") and is especially important in adults — it defines a high-risk subgroup but is now targeted with TKIs
• Other recurrent translocations include t(12;21) involving RUNX1 and ETV6 (25% of childhood B-ALL, favorable)

Acute Myeloid Leukemia (AML)

• Arises from myeloid progenitors; associated with acquired mutations in transcription factor genes (RUNX1, CEBPA, PML::RARA in APL) that block myeloid differentiation
• Complementary mutations in growth-promoting genes: FLT3 (internal tandem duplication — most common, ~30%), NPM1, IDH1/2
• Can arise de novo or evolve from myelodysplastic neoplasms, myeloproliferative neoplasms, or following genotoxic exposures (therapy-related AML)
• The hallmark morphologic feature is >20% myeloid blasts in the bone marrow

Chronic Myeloid Leukemia (CML)

• Virtually defined by the Philadelphia chromosome — reciprocal t(9;22)(q34;q11) — present in >90% of cases
• This translocation creates the BCR::ABL chimeric gene, which encodes a constitutively active 210-kDa tyrosine kinase
• BCR-ABL activates multiple downstream proliferative pathways (RAS, PI3K, STAT5), causing massive clonal expansion of myeloid cells with retained ability to differentiate (hence chronic phase)
• The cell of origin is a pluripotent HSC, explaining why CML can transform to AML or ALL (blast crisis)
• CML follows a tri-phasic course: chronic phase → accelerated phase → blast crisis

Chronic Lymphocytic Leukemia (CLL)

• The most common adult leukemia in Western countries
• Clonal proliferation of mature B cells expressing CD5, CD19, CD20 (dim), CD23
• Key molecular features: deletion 13q14 (most common; favorable), trisomy 12, deletion 11q (ATM loss), deletion 17p (TP53 loss — worst prognosis), IGHV mutation status (mutated = better prognosis)
• Cells accumulate due to resistance to apoptosis (overexpression of BCL-2) rather than rapid proliferation

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4. Morphology and Diagnosis

4.1 Acute Leukemias (AML & ALL)

• High nuclear-to-cytoplasm ratio
• Fine (euchromatic) chromatin
• Prominent nucleoli
• Scant cytoplasm
• Mitotic figures

Diagnostic threshold: Acute leukemia is defined by ≥20% blasts in the marrow (WHO criteria).

4.2 CML

• Peripheral blood smear shows leukocytosis (often 50,000–200,000/μL) with the full spectrum of myeloid maturation: blasts, promyelocytes, myelocytes, metamyelocytes, bands, segmented neutrophils
• Basophilia and eosinophilia are characteristic
• Splenomegaly is often pronounced (extramedullary hematopoiesis)
• Marrow is hypercellular with near-complete replacement of fat

4.3 CLL

• Peripheral blood: absolute lymphocytosis (>5,000/μL) of small, mature-appearing lymphocytes with "smudge cells" (fragile lymphocytes crushed on the smear) — pathognomonic
• Bone marrow: small lymphocytes with condensed, clumped nuclear chromatin; scant cytoplasm

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5. Clinical Features

5.1 Symptoms Resulting from Bone Marrow Failure

5.2 Infiltrative Features

5.3 CML-Specific Course

5.4 CLL-Specific Features

• Often indolent; incidentally discovered on routine CBC
• Rai staging (0–IV) based on lymphocytosis, lymphadenopathy, organomegaly, anemia, thrombocytopenia
• Complications: hypogammaglobulinemia → infections; autoimmune hemolytic anemia; Richter transformation (→ aggressive DLBCL)

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6. Laboratory Investigations

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7. Treatment (Pharmacological Focus)

7.1 ALL

• Highly curable (~85–90% in children) with multi-drug combination chemotherapy
• Standard phases: Induction → Consolidation → Maintenance

• Imatinib (1st gen), dasatinib (2nd gen — preferred due to CNS penetration), or ponatinib (3rd gen)

7.2 AML

• Cytarabine (100–200 mg/m²) continuous IV infusion × 7 days
• Daunorubicin or idarubicin × 3 days
• Goal: achieve complete remission (marrow blasts <5%)

• Defined by t(15;17) → PML::RARA fusion
• Treated with All-Trans Retinoic Acid (ATRA) + Arsenic trioxide (ATO) — not conventional chemotherapy
• ATRA induces differentiation of the arrested promyelocytes; ATO promotes apoptosis
• Cure rate ~90% — the most curable form of AML

• FLT3 inhibitors: Midostaurin (FLT3-ITD/TKD+), gilteritinib (R/R AML)
• IDH inhibitors: Ivosidenib (IDH1 mut), enasidenib (IDH2 mut)
• BCL-2 inhibitor: Venetoclax + azacitidine (for older/unfit patients)
• Gemtuzumab ozogamicin: Anti-CD33 antibody-drug conjugate (CD33+ AML)

7.3 CML — The Paradigm of Targeted Therapy

7.4 CLL

• Many patients require no immediate treatment ("watch and wait")
• Treatment indicated for symptomatic or advanced disease

• Ibrutinib: Irreversible BTK (Bruton's tyrosine kinase) inhibitor → blocks B-cell receptor signaling → CLL cell death
• Venetoclax: BCL-2 inhibitor → restores apoptosis
• Rituximab/Obinutuzumab: Anti-CD20 monoclonal antibodies → ADCC + complement-mediated cytotoxicity

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8. Complications

8.1 Tumor Lysis Syndrome (TLS)

• Occurs with rapid cell death (especially after chemotherapy in ALL/AML)
• Massive release of intracellular contents: hyperuricemia, hyperkalemia, hyperphosphatemia, hypocalcemia → AKI, arrhythmias, seizures
• Prevention: Allopurinol or rasburicase (recombinant urate oxidase) + vigorous IV hydration before therapy

8.2 Disseminated Intravascular Coagulation (DIC)

• Particularly associated with AML-M3 (APL)
• Leukemic promyelocytes release tissue factor and granule contents → activation of coagulation cascade → consumptive coagulopathy
• Can be fatal if ATRA/ATO therapy is delayed

8.3 Differentiation Syndrome (formerly ATRA syndrome)

• Complication of ATRA/ATO treatment in APL
• Symptoms: fever, respiratory distress, weight gain, effusions, hypotension (due to cytokine release from differentiating cells)
• Management: high-dose dexamethasone

8.4 Infections

• Major cause of morbidity and mortality in all leukemias
• Prolonged neutropenia from chemotherapy → bacterial, fungal (Aspergillus, Candida), and viral infections
• Prophylaxis: antibacterial (fluoroquinolones), antifungal (fluconazole/posaconazole), antiviral (acyclovir)

8.5 Drug-Specific Toxicities (Pharmacy-Relevant)

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9. Prognosis

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10. Summary Table: Four Major Leukemias

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11. Key Concepts for Pharmacy Students

1. Imatinib as a paradigm: The development of imatinib against BCR-ABL in CML transformed a fatal disease into a chronic, manageable condition and validated the concept of molecular targeted therapy. It demonstrates how understanding oncogenic kinases enables rational drug design.
2. ATRA in APL — differentiation therapy: Instead of killing cells, ATRA forces maturation arrest to resolve. This highlights that restoring differentiation, not just inducing cytotoxicity, is a valid pharmacological strategy.
3. TKI resistance: Mutations in the BCR-ABL kinase domain (e.g., T315I "gatekeeper mutation") confer resistance to 1st- and 2nd-generation TKIs → drives development of ponatinib and asciminib, illustrating pharmacological evolution against resistance.
4. Tumor Lysis Syndrome prophylaxis: Pharmacists play a critical role in risk-stratifying patients and ensuring allopurinol/rasburicase is prescribed before cytotoxic therapy is initiated.
5. Drug interactions with TKIs: Imatinib, dasatinib, nilotinib are substrates and inhibitors of CYP3A4 — significant drug interactions with azole antifungals, rifampin, and other CYP3A4 modulators. Pharmacist counseling is essential.
6. BCL-2 inhibition + TLS risk: Venetoclax carries a high risk of TLS, especially in CLL with high tumor burden. A mandatory dose ramp-up schedule (5 mg → 10 mg → 20 mg → 50 mg → 100 mg over 5 weeks) is required.

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References

• Robbins & Kumar Pathologic Basis of Disease, 10th Edition — Chapter 13: Diseases of White Blood Cells, Lymph Nodes, Spleen, and Thymus, pp. 556–590
• Katzung's Basic and Clinical Pharmacology, 16th Edition — Chapter on Antineoplastic Agents
• WHO Classification of Haematopoietic and Lymphoid Tumours, 5th Edition (2022)

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